There's a tendency to default to what I think of as the 'declaration' scene. The lead character, after waffling for three hundred pages, suddenly announces a single choice, leaving the others to nobly accept it. I've always found that ending flat, honestly. It feels like an erasure of the emotional groundwork built with the other characters. A better approach, though less common, is the 'redefinition' route. The group dynamic doesn't vanish; it shifts. Maybe they become a found family or a tight-knit support system where romantic love for one doesn't negate profound, complex bonds with the rest. I see this more in web serials than in traditional publishing. 'The Pale Court' web novel did something interesting where the protagonist didn't 'choose' in a traditional sense but forged a new political union that bound them all together in a different, arguably more intimate way. The conflict resolution becomes less about picking a winner and more about restructuring the relationships entirely, which for me, is a lot more satisfying than a clean-cut, monogamous bow on a messy situation.
That said, the power fantasy of 'why choose' is its own valid endgame. Sometimes readers just want the fantasy to be validated, and the conflict resolution is everyone agreeing to share happily. It can feel shallow, but if the story's tone was light and comedic from the start, it fits. The real narrative failure happens when a story built on genuine emotional conflict between well-developed characters tries to slap on a polyamorous solution in the last chapter without any of the necessary communication or boundary-setting shown on page. It rings false.